Streptococcus mutans Iron Uptake/Transport Affects Formation of the Plaque Biofilm and Cariogenesis

Iron is an essential micronutrient for most bacterial pathogens, but its restricted availability in the human host forms a nutritional barrier to infection. Streptococcus mutans, the primary acidogenic component of dental plaque, uses a FimA lipoprotein to rob iron from host proteins. We determined that expression of the FimA transporter is iron-responsive and subject to repressible control by Dlg, a metalloregulatory protein for which there is an iron-dependent homolog (DtxR) in Corynebacterium diphtheriae. We propose that Dlg regulates genes belonging to a putative S. mutans iron “stimulon", and that such regulation can influence biofilm architecture and cariogenesis. Objectives: 1) to elucidate a role for Dlg and iron in formation of the plaque biofilm and cariogenesis, and 2) to identify additional S. mutans genes that are subject to Dlg control. To foster these studies, we used the S. mutans UA130 wild-type strain to generate knockout mutations in fimA (GMS700) and dlg (GMS800), respectively. Methods: Crystal violet release assays were used to quantitate S. mutans wild-type and mutant biofilms formed on abiotic surfaces, and scanning electron microscopy was performed to visualize biofilm structure. We also monitored the cariogenic potential of these biofilms in germfree rats, and analyzed UA130 and GMS800 cDNAs by differential display PCR (ddPCR) to identify genes that are Dlg regulated. Results: The GMS700 iron uptake mutant was adherence-deficient in biofilm assays and hypocariogenic in germfree rats. GMS800, also hypocariogenic in this animal model, formed biofilms with increased biomass and a distinct architecture relative to wild-type. The results of ddPCR revealed multiple S. mutans genes that are subject to Dlg control. Conclusions: These findings support a Dlg regulon in S. mutans and implicate iron in S. mutans biofilm formation and cariogenesis. This research was supported by NIH Grant R03DE12270, the Vermont Genetics Network and the Middlebury College Biology Department.